The present invention relates to an objective lens and an optical pickup apparatus.
In recent years, a trend toward a short wavelength of a laser light source used as a light source for reproducing information recorded on an optical disc or for recording information on an optical disc has been advanced, in an optical pickup apparatus, and there have been put to practical use laser light sources with a wavelength of 405 nm such as, for example, a violet semiconductor laser and a violet SHG laser wherein generation of the second harmonic is used for wavelength conversion of an infra-red semiconductor laser.
If these violet laser light sources are used, information in 15-20 GB can be recorded for an optical disc having a diameter of 12 cm, when using an objective lens with a numerical aperture (NA) identical to that of a digital versatile disc (which will be abbreviated as DVD, hereafter), and when NA of an objective lens is enhanced to 0.85, information in 23-27 GB can be recorded for an optical disc having a diameter of 12 cm. Hereafter, optical discs and magneto-optical discs which use a violet laser light source are generically called “a high density optical disc”.
Incidentally, two standards are presently proposed for high density optical discs. One of them is a Blu-ray disc (which will be abbreviated as BD, hereafter) which employs an objective lens with NA 0.85 and has a protective layer whose thickness is 0.1 mm, and the other is HD DVD (which will be abbreviated as HD, hereafter) which employs an objective lens with NA 0.65-0.67 and has a protective layer whose thickness is 0.6 mm. When considering a possibility that these high density optical discs in the two standards appear on the market, a compatible optical pickup apparatus that can conduct recording and reproducing for both of the aforementioned high density optical discs as well as for the conventional DVD and CD is important, and it is especially preferable that compatibility is attained by an objective lens. The expression “compatibility is attained by an objective lens” in this case means that, when rays of light which are different each other in terms of only a wavelength enter the objective lens at the same angle, each ray of light can be converged on a recording surface of the optical disc corresponding to each wavelength.
On the other hand, in the optical pickup apparatus, it is required that the optical pickup apparatus operates safely, even when an individual difference of a laser to be incorporated causes the dispersion of an oscillation wavelength (wavelength characteristics), or even when instantaneous wavelength changes (chromatic aberration) representing the so-called mode-hop or gentle wavelength changes (temperature characteristics) originating from a rise in ambient temperature are caused during operations. For the purpose above mentioned, it is necessary that there is no deterioration of wavefront aberration of the objective lens for the changes in ambient conditions.
A conventional DVD-CD compatible lens is a diffractive lens wherein compatibility is attained by using diffraction actions even for a light source having a wavelength difference such as that between 655 nm and 785 nm, and aberration deterioration is controlled for environmental characteristics which are accompanied by wavelength changes, such as chromatic aberration, wavelength characteristics and temperature characteristics. However, if this technology is applied on compatibility for high density discs and conventional discs, the following problem takes place. The problems is that there is no solution which satisfies both of compatibility and ambient characteristics even when designing with one parameter of wavelength-dependence of diffraction, for the lens compatible for high density discs and conventional discs, while, the wavelength-dependence of the diffraction determined to attain compatibility just satisfies ambient characteristics, in the DVD-CD compatible lens.
In the invention described in TOKKAI No. 2002-298422, there is known the technology to use an objective optical system on which a diffractive structure is provided, and to attain compatibility under an optical system magnification that is zero, in each of HD and DVD.
In the invention described in TOKKAI No. 2002-298422, there arises a problem wherein wavelength characteristics caused by diffraction grow greater, although coma caused by lens shift in the case of tracking is not generated in this method, and wavefront aberration is deteriorated when an oscillation wavelength in a certain lot of lasers is different from that in another lot.